In U.S. Pat. No. 3,969,159 and elsewhere, casting powders are described which contain, as a principal component, lime-alumina-silicates and which can contain fluxing agents such as alkali-metal compounds, e.g. alkali-metal carbonates. The casting powders of the prior art may also have, as a casting powder component, bituminous coal flyash.
Such casting powders are generally applied to the surface of a steel melt prior to or concurrently with casting in ingot molds or continuous casting molds so as to form an insulating layer between the melt and the mold, to improve the surface characteristics of the cast body, and to protect the mold from erosion by the molten metal.
A casting powder, for such purposes, must have two principal characteristics. Firstly, the casting powder must be readily melted at the interface or boundary layer between the molten casting material, i.e. the liquid steel, and the powder. This permits the casting-powder components to form a continuous film upon the molten metal. Secondly, the casting-powder layer itself must have high insulating effect, i.e. must be capable of restricting heat transfer by conduction.
In conventional casting powders, these characteristics are obtained by providing the casting powder in extremely fine-grain form, i.e. in particle sizes substantially lower than 60 microns. This, however, has created a major problem since particle sizes below this threshold, i.e. below the dust limit, are readily entrained into the atmosphere and produce substantial environmental-contamination hazards. They are detrimental to the health of the personnel of metallurgical plants in which they are used.
Prior attempts to eliminate the problem have proved to be unavailing since any increase in the particle size of conventional casting powders reduces the ability of the powder to melt at the interface and form a film or limits the thermal insulating properties.